This project deals with two topics: The first is the design, assembly, and testing of a new flowmeter that only requires the insertion of a commercially available multi-probe catheter and is expected to be suitable for the direct measurement of cardiac output in man. The second concerns analysis of a novel hypothesis regarding the role played by branch points in the systemic arterial system. With respect to flow measurement, we have developed a method whereby a catheter that measures pressure in an artery, e.g., the aorta, continuously at three points separated by several centimeters in the vessel's axial direction, will yield instantaneous volume flow in that artery. Several other quantities of clinical and physiological significance can be derived simultaneously. These include vessel internal radius, pulse wave velocity, the degree to which the vessel wall is viscoelastic rather than elastic, the vessel's characteristic impedance, and the global reflection coefficient as seen from the vessel under study. If the selected artery is the ascending aorta, stroke volume, cardiac output, and the load impedance presented by the vascular system to the ventricle can be determined. The many branch points in the systemic arterial system have generally been acknowledged to serve in the distribution of blood from the ventricle to the immediate vicinity of the cells in the peripheral organs. The hypothesis is advanced here that their role is far more sophisticated by serving to linearize the arterial system for both steady and pulsatile components of pressure and flow. The validity of this hypothesis will be analyzed by solving the complete Navier-Stokes equation for actual branch points in the arterial system, and experimental verification of the results. If valid, the concept has far reaching consequences for our understanding of the operation of the arterial system. These may include conditions that disturb linearity as the cause for the preferred formation of plaques distal to branch points.